Hard surfaced steel plates by arc welding have been employed for wear parts protection. Such plates have been manufactured by open-arc welding or by using submerged welding procedures. On the other hand, metal-sprayed procedures have been used, including fusing sprayed alloy powders, these procedures being employed for the same purpose.
In this connection, reference is made to U.S. Pat. No. 4,471,034 which relates to the production of alloy coatings on cast iron parts by thermal spraying using plasma transfer arc welding. The alloys disclosed are nickel-base self-fluxing, self-bonding alloys.
Another patent is U.S. Pat. No. 4,806,394 which relates to a method for producing a wear-resistant layer on a metal base using autogenous flame spraying to produce a bonded layer of at least 1 mm.
A disadvantage of the foregoing methods is the limitation on the thickness of the coating or the tendency for substantial dilution of the coating metal with the base metal. Because of the tendency of dilution of the coating, it is difficult to retain the primary composition of the alloy coating and, because of the reactions in the flame, the hardness expected of the self-fluxing, self-bonding alloy.
In addition, it is sometimes necessary to apply the coating in multiple passes on the base metal in order to assure the required hardness. This is usually attended by certain inconveniences such as the following: (1) additional welding labor costs; (2) increased production hours, (3) the decrease in hardness approaching the metal substrate because of dilution; (4) the difficulty of controlling thickness together with the dilution of the coating because of overheating; and (5) the fact that the final coated plate does not always meet specifications.
Moreover, in carrying out the self-bonding powder fusing procedure of the prior art, it is very difficult to produce a thick layer of the coating with the desired hardness and because of surface cracking and lifting of the coating. Thick coatings are especially desirable for wear parts produced from metal plates, particularly where long life of the coated substrate is essential.
Another disadvantage of producing coated metal substrates by thermal spraying is considerable loss of powder during spraying which adds to the cost of producing the final product.
The invention will now be described in light of the following disclosure, the drawings and the appended claims.